Microsoft Corp. renewed its push to catch up with Apple Inc. and Google Inc. in mobile devices, unveiling updated Windows Phone software with voice-search features that its rivals already offer.

At its Build conference in San Francisco Wednesday, Microsoft showed off a voice-controlled digital assistant called Cortana that is similar to Apple's Siri. Cortana is part of new Windows Phone 8.1 software for mobile gadgets, said Vice President Joe Belfiore.

Chief Executive Officer Satya Nadella, who was appointed to lead the world's largest software maker in February, is working to remake Microsoft for an era where smartphones and tablets have become central. To do so, he must strike a delicate balance between offering Microsoft's software for competing platforms while still keeping the company's Windows operating system as a core focus. Wednesday's event was designed to re-emphasize how Windows remains important for the company.

"Reinvigorating the Windows franchise continues to be a key ingredient in Microsoft's recipe for success as it looks to offset headwinds from the secular challenges in the PC market," Daniel Ives, an analyst at FBR Capital Markets & Co., wrote in a note.

Contrasting events

Nadella's tricky balancing act was underscored by contrasting Microsoft events last week and this week. The CEO last week introduced Microsoft Office software for Apple's iPads, and said he'll "hold nothing back" to get the company's programs across all devices, in a clear departure from the software maker's longtime focus on Windows.

Yet this week's Build conference puts Windows front-and-center by giving developers tools and tactics to boost Microsoft's single-digit share in phones and tablets, as well as providing money-making opportunities in a shrinking personal-computer market.

Belfiore said Wednesday that the new Windows Phone 8.1 will have an action center, which is a pull-down menu that gives people access to basic functions regardless of whether programs are already open.

Custom designs

The software will be available to customers who already have Windows phones in the next few months and be installed on new devices late this month and in early May. It also lets users create custom lock screens and offers new choices for designing the start screen.

Cortana, which "fully replaces the search function on Windows Phone," according to Belfiore, is named for the artificial-intelligence character in Microsoft's best-selling "Halo" Xbox video game series.

Developers outside Microsoft can write their own apps for Cortana, Belfiore said. With users' permission, Cortana can learn about them from their searches, keep track of people they interact with and set up an "inner circle" of the most important people from those contacts, and determine who can contact users during the times of day they set as "quiet hours."

Belfiore demonstrated how he allowed Cortana to scan his e-mail, causing the program to ask him whether to track an Alaska Airlines flight mentioned in an e-mail itinerary.

The program can be used to start a Skype call, add a TV show to the user's Hulu queue or check a friend's Facebook posts.

"She's great at helping me get things done, whether it's by talking or typing," Belfiore said.

Windows growth

Windows stands to be the fastest-growing smartphone operating system over the next four years with 30 percent annual growth, according to a February report by researcher IDC. Even at that rate, Windows Phone would only make up 7 percent of the total market in 2018, compared with Google's Android software at 76 percent and Apple's iOS at 14 percent.

In tablets, Windows had 3.4 percent share in 2013, IDC said. For Microsoft's Surface tablet, the share was 1 percent.

Global computer shipments fell a record 10 percent last year and are forecast to continue to decline this year as tablets and smartphones lure consumers away from traditional desktop and notebook designs, according to IDC. Tablet sales volume - dominated by Apple and Google, whose operating systems account for 95 percent of that market - will rise an average 16 percent annually through to 2017, the market researcher said.

One possible boon to Windows developers: Microsoft is working on obtaining tools, via acquisition or development, to make it easier for developers to design apps for different operating systems without having to rewrite software. The company is considering an acquisition or investment in Xamarin Inc., or other companies that enable mobile apps to run on different devices, people with knowledge of the matter said last month. Microsoft may also build those capabilities itself, one of the people said.